1. Field of Invention
This invention relates to the field of generating electricity, steam or hot water, chemicals and gaseous fuels from solid or liquid fuels. It also relates to the field of using photosynthesis to produce oxygen and algae from carbon dioxide and water.
2. Prior Art
Coal and other solid and liquid fuels have been gasified into more readily usable gaseous fuels for over one hundred years. In the most common reaction, the char-steam reaction, solid carbon reacts with steam to produce carbon monoxide and hydrogen. This reaction, being endothermic, requires heat and high temperatures to proceed. Traditionally, air was supplied to the fuel to partially burn the fuel and supply heat and temperature needed to drive the char-steam reaction.
Air-blown gasifiers (gasifiers using oxygen in air as an oxidizer for combustion) produce a low heat content fuel (synfuel) containing the nitrogen from the air. This fuel is burned with additional combustion air to produce heat. This fuel, and/or the combustion gases resulting after combustion, must be cleaned or treated to avoid air pollution. This is particularly difficult because of the high volumes of gas involved. The nitrogen in the air increases the volume of fuel gas that must be cleaned, and to reduce energy consumption of the air compressors, the gasification process proceeds at low pressures, which increases the specific volume of the fuel gas.
If an oxygen separation plant is used, oxygen rather than air is sent to the gasifier, which increases the heat content of the synfuel and reduces the volume of synfuel needed to be treated. Currently, this is the preferred configuration of coal or petcoke gasification projects: An IGCC (integrated Gasifier combined cycle) plant comprises one or more ASU (air separation units), oxygen blown gasifiers, combustion gas turbines with heat recovery steam generators and steam turbines. An example of this type of technology is disclosed and claimed in U.S. Pat. No. 4,341,069 (Bell et al.).
A third type of gasifier, the Twin-Fluidized Bed Steam Gasifier, uses steam to partially gasify the carbon-bearing fuel in a fluidized bed gasifier. Because steam does not supply enough heat for the gasification reactions, hot sand is also added to the gasifier. The cooled sand and residual ash from the carbon-bearing material is blown out of the gasifier and separated from the gas in a gasifier cyclone. This material drops into a fluidized bed combustor chamber, along with combustion air. The combustion air burns the residual carbon in the ash and reheats the sand. The ash and sand is removed from the flue gas in a cyclone. After separating the ash from the sand the sand returns to the gasifier.
This particular type of gasification has been used in the petrochemical industry to gasify petroleum coke for over fifty years as the Exxon Flexicoker process. A similar device for gasifying biomass has recently been developed by Battelle National Laboratories and has been commercially offered as the Silvagas process. A similar process has been offered by Taylor Biomass Industries. The advantage of this type of gasifier is that it produces high energy content fuel gases (gases with little nitrogen in them) without the expense of a air separation unit.
To further increase system efficiency, several recent designs incorporate fuel cells. Two different types of fuel cells have been considered in recent fuel cell-gasification patents: the Solid Oxide Fuel cell (SOFC), and the molten carbonate fuel cell (MCFC).
The SOFC uses a ceramic, solid-phase electrolyte operating at about 1830° F. Oxygen reacts with the cathode in the reaction:O2+4e−→2O=The oxygen ions diffuse through the electrolyte and react with the fuel gas at the anode in the reactions:H2+O=→H2O+2e−CO+O=→CO2+2e−andCH4+4O=→2H2O+CO2+8e−The electrons travel from anode to cathode through an electric load, producing power. Because of the high temperatures, hydrocarbons in the fuel stream can also be reformed into hydrogen and carbon monoxide, if sufficient water vapor is available in the fuel stream.
The MCFC uses a molten carbonate salt mixture operating at about 1200° F. as an electrolyte. Oxygen and carbon dioxide reacts with the cathode in the reaction:O2+2CO2+4e−→2CO3=The carbonate ions diffuse through the electrolyte and react with the fuel gas at the anode in the reactions:H2+CO3=→H2O+CO2+2e−andCO+CO3=→2CO2+2e−
As with the SOFC, the electrons travel from anode to cathode through an electric load, producing power. Because of the high temperatures. hydrocarbons in the fuel stream can also be reformed into hydrogen and carbon monoxide, if sufficient water vapor is available in the fuel stream. Unlike the SOFC, the oxidant stream must contain carbon dioxide, which must be recovered from the system.
Because both the SOFC and MCFC reaction products accumulate in the anode, fuel cannot be completely consumed in the anode; eventually, the reaction products dilute the concentration of fuel enough so that the fuel cell potential decreases below economically recoverable voltages. At that point, the remaining fuel is generally burned in an external combustor to produce heat that pre-heats the gases entering the fuel cell. The heat can also be used to produce steam to produce additional power or heat.
One example of a SOFC gasification technology is disclosed and claimed in U.S. Pat. No. 5,955,039 (Dowdy), which is incorporated by reference herein in its entirety. In this invention the compressor section of a gas turbine provides pressurized air to both a fuel cell and to a Gasifier. A portion of the oxygen in the air entering the fuel cell reacts with the cathode to produce water, and this vitiated air stream then enters a combustor, where it reacts with fuel leaving the Gasifier. The other portion of the compressed air enters the Gasifier where it reacts with fuel to form hydrocarbons, carbon monoxide and hydrogen. After cleaning and phase shifting some of the carbon monoxide to hydrogen, some of the hydrogen is sent to the fuel cell and the remainder of the gas is burned in the combustor. This reaction produces a hot, pressurized gas stream which drives the turbine section of the gas turbine. One disadvantage with this prior art is that the Nitrogen in the exhaust forms NOx when it is combusted. A second disadvantage is that the nitrogen adds volume to the gas leaving the Gasifier, increasing the expense in cleaning it.
Another example of an prior art, suitable for both SOFC and MCFC equipment is disclosed and claimed in U.S. Pat. No. 6,187,465 (Galloway), which is incorporated by reference herein in its entirety. The all embodiments of this patent feed the fuel cell cathode exhaust to the Gasifier after removing the water from the stream. This arrangement has the disadvantage of requiring a separate oxygen supply to the Gasifier.
Another example of a prior art, is disclosed and claimed in U.S. Pat. No. 5,554,453 (Steinfeld), which is incorporated by reference herein in its entirety. This patent recycles a portion of the Gasifier exhaust back to the Gasifier, supplying the heat needed for gasification by heating this stream with an exhaust stream formed by burning the exhaust of a MCFC with air. This patent requires an extremely high temperature heat exchanger to supply the gasification heat.
The prior art referenced above all produce significant volumes of exhaust to the atmosphere, which must be further cleaned. They also require cooling or heating of streams entering the Gasifier, reducing gasification efficiency. They did not make use of the hot oxygen leaving the fuel cell in the Gasifier.
One example of a prior art is disclosed and claimed in U.S. Pat. No. 4,143,515 (Johnsen) which is incorporated by reference herein in its entirety. This patent did allow oxygen leaving the fuel cell to be directly used in the Gasifier. However, the oxygen and hydrogen used in the fuel cell were generated electrically from water using electrical power.
Another example of prior art is disclosed and claimed in patent application Ser. No. 10/705,289 (Radovich), which is incorporated by reference in its entirety. This patent combines the streams from the anode and cathode into a single gasifier, rather then directing these streams to separate gasifiers and combustors.
A final example of a prior art is disclosed as a Power Point presentation entitled “Coal System Studies: Effects of Methane Content and High-Efficiency Catalytic Gasification” attached to this application. This presentation describes a thermal cycle with the anode exhaust directly connected to a catalytic gasifier. The exhaust from the cathode is directly connected to a turbo-expander.